Process for the removal of nitrile using a selective adsorbent

ABSTRACT

An adsorbent for selective removal of nitrile from a hydrocarbon feedstock includes a substantially homogeneous mixture of a cationic nonacidic zeolite and an inorganic oxide matrix, the zeolite having an accessibility index of between about 0.1 to about 0.4, and the adsorbent being capable, when contacted with a hydrocarbon feedstock containing nitrile and containing dienes in an amount of at least about 1.0% by volume, of adsorbing at least about 80% of the nitrile during contact with at least about 200 volumes of the feedstock per volume of adsorbent.

This is a Division of application Ser. No. 08/529,759, filed Sept. 18,1995.

BACKGROUND OF THE INVENTION

The invention relates to an adsorbent for selectively removing nitrilefrom a hydrocarbon feedstock, to a method for making the adsorbent, andto a process for removing nitrile from a hydrocarbon feedstock using theadsorbent.

Nitrile compounds are frequently undesirable contaminants in feedstockswhich otherwise contain valuable fractions. Particularly, C4 and C5 cutsfrom FCC units can be useful in the preparation of oxygenates such asMTBE, ETBE, TAME and the like. However, these feedstocks typicallycontain nitrites and diolefins. The diolefins may typically behydrogenated before and/or at the etherification reactor. Nitriles,however, must be removed upstream of the reactor and therefore must beremoved in the presence of dienes which make the feedstock higlyreactive.

Upstream nitrile removal techniques include washing, but propionitrilecannot be removed through washing. In the presence of hydrogenationcatalysts, however, propionitrile is believed to undergo conversion tototally or partially hydrogenate products, which behave as basicnitrogen and rapidly poison the acid sites of the catalyst. It isapparent that the need remains for a treatment for removal of nitrilecompounds, especially propionitrile, wherein the agent used for removalis selective to nitrile and has longevity toward the selective removalof nitrile especially in the presence of relatively large amounts ofdienes.

It is therefore the primary object of the present invention to provide anitrile selective adsorbent which is not rapidly deactivated during use,even in the presence of dienes.

It is a further object of the present invention to provide a method forpreparing an adsorbent according to the present invention.

It is a still further object of the present invention to provide aprocess for treating a feedstock such as an FCC etherification feedstockwith a nitrile selective adsorbent according to the invention so as toeffectively and selectively remove nitrile compounds.

Other objects and advantages will appear herein below.

SUMMARY OF THE INVENTION

In accordance with the invention, the foregoing objects and advantagesare readily attained.

An adsorbent is provided according to the invention which comprises asubstantially homogeneous mixture of a cationic nonacidic zeolite and aninorganic oxide matrix, said zeolite having an accessibility index ofbetween about 0.1 to about 0.4, and said adsorbent being capable, whencontacted with a hydrocarbon feedstock containing nitrile and containingdienes in an amount of at least about 1.0% by volume, of adsorbing atleast about 80% of said nitrile during contact with at least about 200volumes of said feedstock per volume of adsorbent.

A method for making the adsorbent of the present invention is providedwhich method comprises the steps of providing a cationic nonacidiczeolite; providing an inert inorganic oxide matrix; washing said matrixwith an alkaline hydroxide so as to provide a washed matrix havingneutralized surface acidity; calcining said washed matrix so as toprovide a calcined matrix having reduced surface hydroxide; mixing saidzeolite and said calcined matrix so as to provide a substantiallyhomogeneous mixture; forming said substantially homogenous mixture intoadsorbent elements; and calcining said adsorbent elements so as toprovide said adsorbent.

A process for treating a hydrocarbon feedstock for removal of nitrile isprovided which process comprises the steps of providing a hydrocarbonfeedstock having a nitrile content of less than or equal to about 400ppm; providing an adsorbent for selective removal of nitrile from ahydrocarbon feedstock comprising a substantially homogeneous mixture ofa cationic nonacidic zeolite and an inorganic oxide matrix, said zeolitehaving an accessibility index of between about 0.1 to about 0.4; andcontacting said feedstock and said adsorbent at nitrile adsorptionconditions whereby at least about 80% of said nitrile content isadsorbed by said adsorbent during contact with at least about 200volumes of said feedstock per volume of said adsorbent.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of preferred embodiments of the inventionfollows, with reference to the attached drawings, wherein:

FIG. 1 is an illustration of propionitrile adsorption isotherms forExample 2;

FIG. 2 is an illustration of propionitrile desorption isobars forExample 3;

FIG. 3 is an illustration of 1,3-pentadiene desorption isobars forExample 3;

FIG. 4 is an illustration of adsorption isotherms on Na zeolites forExample 4;

FIG. 5 is an illustration of adsorption isotherms on K zeolites forExample 4; and

FIG. 6 is an illustration of results of the pilot plant test of Example5.

DETAILED DESCRIPTION

The invention relates to an adsorbent for selectively removing nitrilefrom a hydrocarbon feedstock, to a method for preparing a nitrileselective adsorbent, and to a process for treating a hydrocarbonfeedstock with a nitrile selective adsorbent for removal of nitrilecontaminants contained therein.

According to the invention, a nitrile selective adsorbent is disclosedwhich comprises a substantially homogeneous mixture of a nonacidiccationic zeolite and an inorganic oxide matrix, preferably including abinder and being extruded or otherwise formed into a desired shape foruse in hydrocarbon contacting processes. The zeolite is preferablyselected so as to provide a high activity toward the adsorption ofnitrile, and a low activity toward polymerization of dienes, while thematrix is preferably selected so as to have a high activity towardreversible adsorption of dienes, and a low activity toward theadsorption of nitrile. This combination of properties according to theinvention provides the adsorbent with an advantageous and desirableselectivity toward the adsorption of nitrile compounds such aspropionitrile when contacted with a nitrile containing feedstock, with areduced tendency toward polymerization of dienes which can result infouling of the adsorbent. Further, the activity of the adsorbent towardselective adsorption of nitrile is not rapidly deactivated during use.

The zeolite material of the adsorbent according to the invention ispreferably selected from the group consisting of faujasite, mordenite,offretite, A-zeolite, erionite, L-zeolite, ST-5 zeolite, X-zeolite,Y-zeolite, and mixtures thereof. The zeolite is preferably nonacidic,and preferably is transformed or otherwise treated to contain cationsselected from the group consisting of Li, Na, K, Cs, Mg, Ca, Sr, Ba andmixtures thereof, most preferably Na, K or mixtures thereof. The zeoliteis also preferably selected and prepared so as to have an accessibilityindex, which will be defined below, of between about 0.1 to about 0.4.The accessibility index is a gauge of the relative accessibility of theactive pore sites of the zeolite to nitrile compounds and to othercompounds such as dienes.

The inorganic oxide matrix according to the invention is preferablyselected from the group consisting of silica, alumina, kaolin, clays,ceramics and mixtures thereof. The matrix material is preferablysubstantially inert, may be crystalline or amorphous, and is treatedaccording to the invention so as to have substantially no surfaceacidity. It is also preferable that the matrix be cost-effective andmoldable.

According to the invention, the zeolite and matrix materials arepreferably present in the adsorbent in a ratio by weight of zeolite tomatrix of between about 1:1 to about 1:100. In this range the amount ofzeolite is further selected based upon the amount and type ofcontaminants involved, and the surface area of the zeolite. Theadsorbent preferably has a surface area of between about 100 to about1000 m² /g, most preferably between about 200 to about 600 m² /g, andhas an overall surface acidity of less than or equal to about 2.7mmol/g. The adsorbent according to the invention is selective toward theadsorption of nitrile compounds and is therefore useful in the selectiveremoval of nitrile contaminants from a nitrile containing feedstock.

As set forth above, the zeolite material of the adsorbent of the presentinvention preferably has an accessibility index of between about 0.1 toabout 0.4. In accordance with the invention the accessibility indexrepresents a ratio of the molecular accessibility volume of the zeolitematerial for 2 parallel pentadienes to the molecular accessibilityvolume of the zeolite for propionitrile. It has been found according tothe invention that adsorbents containing zeolites having anaccessibility index as set forth above are particularly effective in theselective removal of nitrile without diene polymerization as desired.The measurement of molecular accessibility is further illustrated inExample 7 set forth below.

The adsorbent of the present invention is preferably prepared asfollows. A zeolite material is provided, preferably * selected from thegroup consisting of faujasite, mordenite, offretite, A-zeolite,erionite, L-zeolite, ST-5 zeolite, X-zeolite, Y-zeolite and mixturesthereof. The zeolite is preferably nonacidic, and has been transformedor otherwise treated so as to contain cations preferably selected fromthe group consisting of Li, Na, K, Cs, Mg, Ca, Sr, Ba and mixturesthereof, most preferably Na, K or mixtures thereof. These cations havebeen found according to the invention to allow access of nitrilecompounds to the active sites of the zeolite while prohibiting entry ofmore than one molecule such as dienes and thereby preventing thepolymerization of dienes as desired in accordance with the presentinvention.

According to the invention, a matrix material is provided which ispreferably selected from the group consisting of silica, alumina,kaolin, clays, ceramics and mixtures thereof. The matrix isadvantageously treated in accordance with the invention so as to reducethe surface acidity of the matrix substantially to nil. This maypreferably be accomplished by washing the matrix material with analkaline base wash material such as an alkaline hydroxide. The matrix ispreferably washed with the alkaline hydroxide so as to neutralize thesurface acidity of the matrix which has been found to be desirable inproviding an adsorbent having the desired activity and longevity towardthe selective removal of nitrile according to the invention. Accordingto the invention, the alkaline hydroxide wash solution is preferablyselected from the group consisting of NaOH, KOH, NH₄ OH and mixturesthereof. After washing with the alkaline hydroxide, the matrix ispreferably further washed, for example with water, so as to remove anyremaining un-reacted alkaline hydroxide. The washed matrix is thenpreferably dried, for example overnight at a temperature of betweenabout 80° to about 250° C., and then calcined, preferably at atemperature of between about 400° to about 850° C. so as to remove anyremaining hydroxide (OH⁻) on the surface of the matrix.

The washed and calcined matrix is then preferably mixed with the zeoliteso as to provide a substantially homogeneous mixture thereof. As setforth above, the zeolite and matrix materials may preferably be providedin amounts sufficient to provide a ratio by weight of the zeolite to thematrix material of between about 1:1 to about 1:100.

Depending upon the desired form of the adsorbent end product, a bindermaterial may preferably be added to the mixture so that the mixture canthen be extruded or otherwise formed into a desirable shape or form,depending upon the type of process in which the adsorbent is to be used.A suitable binder material is Ludox™ although other binder materials maybe suitable in accordance with the invention.

After extrusion, the extruded adsorbent elements are preferably dried,for example overnight at a temperature of between about 80° to about250° C., and then calcined, preferably at a temperature of between about400° to about 700° C., so as to provide the final adsorbent productaccording to the present invention.

The described method for preparing the adsorbent product according tothe invention has been found to provide an adsorbent material having adesirably low surface acidity, preferably less than or equal to about2.7 mmol/g, as measured by the amount of irreversibly adsorbed pyridinemolecules on the surface of the adsorbent. The measurement of surfaceacidity is further described in Example 1 below. This low surfaceacidity has been found according to the invention to be particularlyadvantageous in providing an adsorbent having the desired selectivitytoward adsorption of nitrile.

The adsorbent product according to the invention is especially useful intreating hydrocarbon feedstock containing up to about 400 ppm ofnitrile, preferably between about 3 to about 1000 ppm as nitrogen, andat least about 1.0% dienes, preferably at least about 2.0% by volume ofthe feedstock. A particularly desirable feedstock is an FCCetherification feed, most preferably a C4-C7 cut of such a feed, whichcontains nitrile and diene contaminants. The adsorbent materialaccording to the invention has been found to have excellent selectivityand longevity toward the removal of nitrile from such feedstocks.

The adsorbent product of the present invention may be used in ahydrocarbon contacting process as follows. A suitable feedstock isprovided, as is an adsorbent according to the invention as set forthabove. The feedstock and adsorbent are preferably contacted according tothe invention in suitable treatment facilities, and under moderatenitrile removal conditions, most preferably at a temperature of lessthan or equal to about 300° C., and a pressure of less than or equal toabout 500 psi. The adsorbent according to the invention has been foundto be capable of adsorption of at least 80% of the nitrile contaminantof a feedstock containing in excess of about 1.0% dienes over atreatment period during which about 200 volumes of feedstock weretreated per volume of adsorbent. Thus, the adsorbent according to thepresent invention has an excellent selectivity and longevity for use inselectively removing nitrile contaminants from feedstocks containingrelatively large fractions of nitrile and diene compounds.

Example 1

This example illustrates the measurement of surface acidity of asodium-Y zeolite and a kaolin matrix in accordance with the invention.The surface acidity is measured by measuring the amount of irreversiblyadsorbed pyridine on samples of the relevant material. The kaolin waswashed with NaOH so as to reduce surface acidity in accordance with theinvention. The zeolite and matrix were pretreated with pyridine at 30°C. and 100° C., and were calcined at various temperatures between about400° C. to about 900° C., as shown in Table 1 below.

                  TABLE 1    ______________________________________                        Irreversible adsorption           Calc. Temp.  (mmol pyridine/g)    Material (°C.)   30° C.                                    100° C.    ______________________________________    Na-Y     500            2.5     1.5             600            2.7     1.7    kaolin   600            0.1     0.02             800            0.1     0.0    ______________________________________

As shown in Table 1, zeolite according to the invention illustrates avery weak acidity which is substantially unaffected by calcinationtemperature, and the matrix material after treatment according to theinvention has substantially no acidity, even at very low pretreatmenttemperatures.

Example 2

This example illustrates the adsorption of propionitrile from testsolutions using two adsorbents according to the invention. Twoadsorbents were prepared according to the invention having 20% wt Na--Yzeolite in kaolin, and having 20% wt K--Y zeolite in kaolin. The sampleswere placed in contact with test solutions containing variousconcentrations of propionitrile in pentane, and were allowed to reachequilibrium. Upon reaching equilibrium, the solutions were analyzedusing gas chromatography and the amount of adsorbed propionitrile wasdetermined. FIG. 1 illustrates the adsorption isotherms for the varioustest solutions. As shown in FIG. 1, both adsorbents effected significantreductions in propionitrile for various loadings of the test solution.Further, the size of the cation, K or Na, did not significantly affectthe adsorption of propionitrile.

Example 3

This example illustrates the desorption of nitrile and diene compoundsfrom zeolites and matrix materials according to the invention. Twozeolite samples and a matrix sample were subjected to treatment of testsolutions containing propionitrile and 1,3-pentadiene. The zeolitesamples were a 15% wt Na--Y zeolite and a 15% wt K--Y zeolite, and thematrix sample was kaolin. After extended contact, these samples wereregenerated using a desorption procedure at various temperatures. Thepropionitrile and 1,3-pentadiene desorption isobars were evaluated fromthe gas phase by thermogravimetric analysis at atmospheric pressure.FIGS. 2 and 3 illustrate the desorption isobars. As shown in FIG. 2, thezeolite samples exhibited an excellent desorption for propionitriles.FIG. 3 shows that the diene adsorption of the matrix is readilyreversible. This example also clearly demonstrates the effect of thecation present in the zeolite in controlling the entrance of largermolecules such as the dienes into the zeolite cage. This activity of thecation serves to give the zeolite its desired selectivity towardadsorption of nitrile. Thus, certain combinations of cation and zeolitemay be selected according to the invention to provide nitrileselectivity depending upon the various sizes of the nitrile, diene andcation molecules and the size of the zeolite apertures. For example, bycomparing FIGS. 1 and 3, it can be seen that substituting K cations inplace of Na cations does not significantly affect nitrile adsorption,but does change pentadiene adsorption, thereby increasing selectivitytoward nitrile.

Example 4

This example illustrates the selective adsorption of nitrile withseveral different zeolites. Several adsorbent samples were prepared andevaluated with test solutions as in Example 2. The samples tested were a15% wt Na--Y zeolite in kaolin, a 15% wt Na-mordenite in kaolin, a 15%wt K--Y zeolite in kaolin, and a 15% K--L zeolite in kaolin. FIGS. 4 and5 illustrate the adsorption isotherms for the various samples tested,and show the effect of using zeolites having different surface areas inaccordance with the invention. FIG. 5 illustrates the advantage of K--Yzeolite over K--L zeolite in this application wherein K--Y zeoliteadsorbs more nitrile than K--L zeolite, with a reduced adsorption ofpentadiene. These examples illustrate that the proper selection ofcation, zeolite and matrix in accordance with the invention can bemanipulated so as to control the selectivity of the adsorbent to nitrileremoval, particularly by avoiding factors that favor dienepolymerization, namely bimolecular diffusion and acidity.

Example 5

This example compares the selectivity for nitrile removal of a 40% wt.Na--Y zeolite in kaolin adsorbent extruded using Ludox™ as a binderaccording to the present invention, and several commercial adsorbents,namely, Selexsorb CD (0.7% diene), INTGARD-A (0.7% diene) and SelexsorbCD (<0.1% diene). The feedstock was a C5-C6 FCC cut having a compositionas shown in Table 2 below.

                  TABLE 2    ______________________________________    Feedstock Composition    ______________________________________    C4             2.1%    C5             20.7%    C6             53.4%    C7             21.6%    diene          2.0%    nitrile        9.6 N ppm    basic nitrogen 1.5 ppm    total sulfur   244 ppm    ______________________________________

The feedstock and the various adsorbents being tested were contacted ina pilot plant under identical conditions, and the results are set forthin FIG. 6. As shown, the adsorbent of the present invention clearlyoutperforms the commercial adsorbents in both selectivity and longevitywith respect to nitrile removal.

Example 6

This example illustrates the preparation of an adsorbent according tothe invention. A fully exchanged Na--Y zeolite and a kaolin matrix areprovided. The kaolin was washed with NaOH so as to neutralize theexisting surface acidity of the kaolin matrix. The neutralized matrixwas then washed with water until remaining un-reacted NaOH was removed.The wet washed kaolin was then dried overnight at temperatures between80° and 250° C., and the dried kaolin was then calcined at temperaturesbetween 400° and 850° C. until all surface OH groups were removed. Thecalcined kaolin was then mixed with the zeolite so as to provide ahomogeneous mixture thereof, and peptized alumina was added to themixture as a binder. The combination of mixture and peptized alumina wasthen extruded so as to provide adsorbent elements of the desired sizeand shape, and the adsorbent elements were dried overnight attemperatures between 80° and 250° C. The dried adsorbent elements werethen calcined at temperatures between 400° and 700° C. to obtain thedesired nitrile selective adsorbent according to the invention.

Example 7

This example illustrates the measurement of the accessibility index forseveral zeolites in accordance with the invention. Three zeolites,namely Y-zeolite, mordenite and L-zeolite, were each evaluated whenloaded with Na and K cations respectively. The molecular accessibilityof these zeolites with respect to propionitrile, pentadiene, I(2 linealpentadiene mol) and II(2 parallel pentadiene mol) was determined usingthe Catalysis (™) program from Biosym Technologies Inc. The results ofthese measurements are set forth below in Tables 3 and 4. Both linearand parallel arrangements for the dienes were considered in order toevaluate the possibility of polymerization. The data set forth in Tables3 and 4 illustrate that the zeolite according to the invention willadsorb more nitrile than dienes, and that the potential for dienepolymerization (by getting at least two diene molecules together) isreduced.

                  TABLE 3    ______________________________________               (# molecules per unit cell)                             Na-    K-               Na-Y  K-Y     Mor    mor  Na-L K-L    ______________________________________    propionitrile (A)                 181     153     21   19   16   16    pentadiene   128     124     19   15   14   12    I(2 lineal pentadiene                 54      53      10   10   9.3  9.7    mol)    II(2 parallel pentadiene                 51      45      5    7    6    4.8    mol) (B)    Accessibility index (B/A)                 0.28    0.29    0.23 0.36 0.38 0.30    ______________________________________

                  TABLE 4    ______________________________________               (# molecules per unit cell)                             Na-    K-               Na-Y  K-Y     Mor    mor  Na-L K-L    ______________________________________    propionitrile (A)                 14.4    11.5    7    6.1  6.8  6.4    pentadiene   10.2    9.3     6.3  4.8  6.1  4.8    I(2 lineal pentadiene                 4.3     3.9     3.3  3.3  3.9  3.9    mol)    II(2 parallel pentadiene                 4.1     3.4     1.7  2.3  2.5  1.9    mol) (B)    Accessibility index (B/A)                 0.28    0.30    0.24 0.38 0.37 0.30    ______________________________________

Thus disclosed is an adsorbent, a method for preparing an adsorbent, anda process for using the adsorbent for selective removal of nitrile froma nitrile containing feedstock wherein the adsorbent has an excellentselectivity and longevity toward the removal of nitrile compounds evenin the presence of relatively large amounts of dienes.

It should also be noted that the characteristics of the adsorbent of thepresent invention are also well suited to use of the adsorbent as acatalyst support.

It is to be understood that the invention is not limited to theillustrations described and shown herein, which are deemed to be merelyillustrative of the best modes of carrying out the invention, and whichare susceptible of modification of form, size, arrangement of parts anddetails of operation. The invention rather is intended to encompass allsuch modifications which are within its spirit and scope as defined bythe claims.

What is claimed is:
 1. A process for selectively removing nitrile from ahydrocarbon feedstock, comprising the steps of:providing a hydrocarbonfeedstock having a nitrile content of less than or equal to about 400ppm; providing an adsorbent for selective removal of nitrile from ahydrocarbon feedstock comprising a substantially homogeneous mixture ofa cationic nonacidic zeolite and an inorganic oxide matrix, said zeolitehaving an accessibility index of between about 0.1 to about 0.4; andcontacting said feedstock and said adsorbent at nitrile adsorptionconditions whereby at least about 80% of said nitrile content isadsorbed by said adsorbent during contact with at least about 200volumes of said feedstock per volume of said adsorbent.
 2. A processaccording to claim 1, wherein said contacting step is carried out at atemperature of less than or equal to about 300° C. and a pressure ofless than or equal to about 500 psi.
 3. A process according to claim 1,wherein said feedstock is an FCC etherification feedstock.
 4. A processaccording to claim 3, wherein said feedstock is a C4-C7 cut.
 5. Aprocess according to claim 1, wherein said feedstock has a diene contentof at least about 1.0% by volume.
 6. A process according to claim 1,wherein said feedstock has a diene content of at least about 2.0% byvolume.
 7. A process according to claim 1, wherein said feedstock has anitrile content of between about 3 to about 5000 ppm as nitrogen ppm.